The invention relates to a contact system for thin film solar cells with an upper semiconductor layer, to which is applied to a front ohmic contact.
Thin film solar cells are essentially composed of an electrically conductive base layer deposited on a carrier, a first p-type or n-type semiconductor layer applied to said base, a second n-type or p-type semiconductor layer arranged thereon, to which an electrically conducting front contact, preferably a grid, is applied. Crystalline or amorphous silicon can be used as a basic material. As compared to a-Si solar cells, it is true that crystalline cells do have a higher efficiency but their manufacture is considerably more expensive and thus more costly than that of the a-Si solar cells. Further, there are copper sulfide-cadmium sulfide solar cells, which although less efficient than the known monocrystalline Si solar cells, likewise offer the considerable advantage of permitting a cost saving fabrication. Therefore, Cu.sub.2 S/CdS solar cells are of increasing interest for the terrestrial field.
The efficiency of a solar cell, among other factors, is also dependent on the fact that there is no barrier layer existing between the front contact and the upper semiconductor layer facing the light source. It must be guaranteed that the part of the surface of the semiconductor layer in contact with the front contact, which very often is made of copper, is not subject to any influences which would impair the efficiency of the solar cell.
In order to prevent this it is known, as far as copper sulfide-cadmium sulfide solar cells with a front contact made of copper are concerned, to apply at first a gold layer to the semiconductor layer facing the light source, which offers a good ohmic contact. However, since the gold layer does not form a permeation barrier to the front side copper contact, this does not prevent copper from diffusing into the copper sulfide layer and consequently changing the stoichiometry of the copper layer. Thus, it is necessary to deposit a nickel layer on the gold layer prior to applying the copper front contact. The nickel layer then serves as a permeation barrier. It is obvious that the fabrication of such a contact system is expensive and cost-consuming so that another contact system would appear to be urgently required, especially for the large-scale manufacture of thin film solar cells.